Not a Pro.

Maybe mimic the black tile behind the tub incorporating a ledge. A towel rack under the window. Some large plants in the corners. Maybe all your need.

Some inspiration:

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White Rock · More Info

I wouldn't do anything behind the tub, as much as I love art in its different forms etc. I like visual proportions as they are now-chandelier, window, tub with its faucet. I wouldn't want to interfere with it too much. Your window is your art...in a sense one really looks at it.

I'd do someting both beautiful and very useful next to the tub-like a side table where one keep soaps/brushes (yeah..they should look good))..maybe a cool thing-y for towels (it's very hard, or else I found it to be real hard, to find something cool and free standing and narrow enough, especially new..new costs some abnormal amount of money. I personally decided to manage without..)..plants are always a good idea, to me at least

One can do a shelf I suppose..but it won't stay empty, right? If you really prefer to, I'd go for smth not full length..assymetrical..I wouldn't want to repeat the window lines.

Quite frankly...I would paint the room. It looks dark and dreary.

I like the first photo njmomma posted with a towel rack and fluffy white towels hanging there. I think that would look great with your paint color (which I like very much).

I really like the towel bar right below the window like that first photo shows as well Planra and maybe a tall glass vase with bath bombs which I know my granddaughters will love when they are over and preferring a bath it is really not to dark of a room I think taking The photo into the windows make it seem darker than it really is.

Thank you all For your wonderful advice. Every one is so helpful here